The present invention relates in general to underground enclosures and more particularly to an underground vault or manhole for housing underground utility units such as transformers, oil switches, or other units for electrical, communication, water, sewer, gas, telephone and cable television equipment.
Heretofore, underground vaults and manholes for utilities were made of reinforced concrete or from an assembly of reinforced concrete and bituminous fiber. Such vaults and manholes generally included a tubular member which was disposed vertically in the soil; a top cap disposed at ground level and seated on the tubular member; a grating or cover plate which was seated on the top cap. The top cap had a central opening and the grating or cover plate could be removed to gain access to the tubular member. Additionally, such vaults or manholes also included baffles and base plates. The tubular member was seated on the base plate and the baffle depended from the top cap. A patent of interest is the patent to Couch et al., U.S. Pat. No. 3,390,225, issued on June 25, 1968, for Underground Electrical Vault.
Such underground vaults and manholes were generally made of heavy reinforced concrete with the tubular body thereof made of a light gauge metal. The metal body was usually corrugated for rigidity. At times the body was formed of a bituminized fiber or cardboard in combination with reinforced concrete top cap. In some instances, the top cap was made of cast iron. Such structures with reinforced concrete and cast iron top caps were usually quite heavy, weighing in the vicinity of 500-700 pounds depending on the size. The installation thereof required special lifting equipment.
Reinforced concrete top caps were subject to chipping and cracking from field handling and from traffic loads. Bituminized fiber bodies were lighter than reinforced concrete bodies, but also were weaker. Hence, they were relatively easily damaged during installation and by unstable soil conditions. Metal bodies were undesirable because of their susceptibility to corrosion and galvanic attack. Also, they presented a safety hazard when used in conjunction with high voltage equipment.